During the last five years, we identified several new proteins that are components of novel G protein signaling pathways. Among them are RGS proteins that not only serve as GAPs for G proteins through their RGS domains, but also bind to other proteins through their unique N- and C-termini. We have concentrated on two RGS proteins, GAIP (RGS19), a GAP for Go_i3, and RGS-PX1, a GAP for Go_s which also has a PX domain that identifies it as it as a SNX protein involved in phosphoinositide signaling. We have obtained results indicating that RGS proteins link G protein signaling to other signaling pathways, including EGF and TrkA growth factor signaling and phosphoinositide signaling, and to other cell processes such as protein degradation. The overall goal of the proposed work is to define the molecular components of the novel signalin! networks we have uncovered and to establish their cellular location, trafficking and functions. The underlyin themes are that RGS proteins are both GAPs and bddging molecules that link signaling events and that these interactions are spatially regulated by ceHuJar distribution and trafficking. We will focus on three specific aims: Specific Aim #1: To assess the role of RGS-PX1 in tinking Go_s signaling and EGF receptor (EGFR) down-regulation at early endosomes. This will be accomplished by assessing the effects of activating the _2- adrenergic receptor and G(zs on EGFR degradation, MAP kinase activation, RGS-PX1 localization and, distribution of Go_s-GFP. We hypothesize that RGS-PX1 serves to link signals generated through activation of I Go_sto EGFR signaling. Specific Aim #2: To determine if GIPC functions to organize signaling complexes that I function in growth factor signaling and trafficking. GIPC, a PDZ protein, binds both GAIP and the TrkA growth I factor receptor. We wUi use deconvolution an(] immunoelectron microscopy, a proteomics approach and RNAi I experiments to investigate GIPC s role in assembiy of signaling and trafficking complexes. Specific Aim #3: I To further define Gczi3-GAIP-GIPC-GIPN signaling networks by biochemical, morphological and bioinformatics I approaches= We will concentrate here on i) isolation of putative signaling complexes containing GAIP, G(zi3,j and GPCR, ii) characterization of GIPN, a novel GAIP-interacting protein that is a putative E3 ligase, and iii) usel of bioinformatics to model God-GAIP-GIPC/GIPN signaling networks. These studies can be expected to I considerably expand knowledge of the role of RGS proteins in linking G protein and growth factor signaling. ! Due to their broad functions in regulation of cell functions, including cell growth, differentiation and mitogenesis, t RGS proteins represent attractive targets for development of pharmacologic and anti-tumor agents. 1